The present invention relates generally to control systems for a plurality of transformer relays and more particularly to control systems for a plurality of transformer relays where the secondary windings of the transformer relays are not balanced or where the primary windings of the transformer relays are connected to separate sources of power.
Transformer relays of the type contemplated to be controlled by the control system of the present invention are available in the art. An example in U.S. Pat. No. 3,461,354, Bollmeier, Magnetic Remote Control Switch, issued Aug. 12, 1969, which describes a magnetically stable transformer relay having primary and secondary coils with the secondary coil being connected to a switch and a rectifier for shorting the secondary coil to allow unidirectional current flow in a desired direction. The control system for the transformer relay is the single rectifier coupled in connection with a double pole, double throw switch which allows the rectifier to be momentarily coupled in either direction across the secondary of the transformer relay. The Bollmeier patent discloses a single transformer relay with a single control switch. Another transformer relay with which the control system of the present invention may be utilized is illustrated in U.S. Pat. No. 4,321,652, Baker et al., Low Voltage Transformer Relay, issued Mar. 23, 1982. Baker also discloses a magnetically stable transformer relay having a primary winding and a secondary winding with the unidirectional flow of current in the secondary winding controlling the state of the transformer relay. The control system disclosed in Baker is a single pole, double throw momentary action switch coupled with a pair of diodes, one in each direction, to allow a unidirectional current flow in the secondary winding of the transformer relay in either direction. The control system in Baker discloses a single transformer relay with a plurality of rectifying switches. The transformer relay in both Bollmeier and Baker are magnetically latched to either of two stable states. The control of the state of the transformer relay is provided by the unidirectional flow of current in the secondary winding (coil). A flow in one direction will control the transformer relay to an "on" state (closing a load switch) and a flow of current in the other direction will cause the transformer relay to be controlled to an "off" state (opening a load switch).
A control system for a transformer relay as described in Bollmeier and Baker is described in U.S. Pat. No. 4,338,649, Mosier, A System for Remotely Controlling a Load, issued July 6, 1982. The control systems described in Mosier provide control of a single transformer relay with a plurality of switches or controls.
Many applications, however, require the control of a plurality of transformer relays with one or more switches positioned at one or more switch locations. While transformer relays can be connected with their secondary windings in parallel, to do so creates certain problems. In a large building or industrial complex, the power source supplying the building may be multi-phase. In this case, the individual transformer relays may be connected to differing phases of the same power source. This, in effect, means that each transformer relay may be connected to a separate power source. If the secondary windings of these transformer relays are then coupled in parallel, undesirable circulating currents between the secondary windings of the transformer relays will occur. This is because the instantaneous voltage between secondary windings of the transformer relays and the balance between those voltages will vary creating the circulating currents between them. These circulating currents may cause inappropriate uncontrolled operation. Even where the transformer relays are all connected to the same power source, i.e., to the same phase, there can still be problems. Since the transformer relays are not exactly matched or balanced, the exact voltage present on the secondary winding of each transformer relay will still vary from transformer relay to transformer relay. Since differing voltages will again occur, currents will again circulate between the secondary windings of the plural transformer relays causing reliability problems.